All this would seem defensive in the extreme. It would appear that she is so eager to avoid being roasted in the media and so adverse to answering questions that she is hiding in plain sight as she is running.
So why bother announcing?
The answer is unbelievable and sickening. It is a lawyer's announcement. Under the campaign finance laws, she cannot pay her staff except out of campaign money and she can't raise that until she becomes an official candidate. So to start the paychecks flowing to her image, hair, makeup, issue, speech, conduct and acting coaches, she has to announce.
She is announcing to comply with the small print of the law while ignoring the big print of public relations. Why? The public be damned. She is unopposed in the Democratic primary and has the party so locked up tightly that she is confident of getting a pass no matter how low her poll numbers go.
That she would think it would be OK to announce in a tweet -- 140 characters, no questions, and no elaborations -- is incredible.
It is of a piece with a private email server. Take the hit now. Don't worry about how it looks. Just play defense until the election comes around. A solution only a lawyer could love.